Conventionally, applications of ultrafiltration membranes to uses in various fields such as production of pure water for electronic industry, recovery of electroplating coatings, treatment of wastes of paper-producing or pulp factories, treatment of oil-containing wastes, recycling of the wastes from buildings, treatment of wastes in atomic power stations, clarification of fruit juices, production of live sake, concentration of whey, desalination, production of concentrated milk, concentration of egg white, treatment of bean proteins, recovery of enzymes, bioreactors, separation of biological products, removal of particles in the air and removal of particles in organic liquids have been proposed and practiced. In the fields where the ultrafiltration membranes are used, such as the fields mentioned above, the following properties concerning the durability of the membrane are usually required:
i) Durability against washing with hot water or sterilization with steam (heat resistance); PA1 ii) Chemical Resistance (resistance to acids, bases, chlorine and so on); PA1 iii) Solvent Resistance; and PA1 iv) Low Fouling Properties (low protein-adsorbing properties).
These properties are required because of the treatment conditions normally applied to ultrafiltration membranes, including sterilization and washing for the recovery of the separated material. Separation efficiency tends to be adversely affected by the concentrated solute layer formed on the surface of the membrane during processing. However, an ultrafiltration membrane meeting these requirements has not yet been developed, so that ultrafiltration membranes with high durability are demanded.
Cellulose acetate is often employed as a material for ultrafiltration membranes because they are easy to process. However, ultrafiltration membranes made of cellulose acetate are not satisfactory in durability such as heat resistance, alkali resistance and solvent resistance. Although ultrafiltration membranes made of polyacrylonitrile and polyvinylidene fluoride (Polymer, vol. 21, p. 1047(1980)) have improved solvent resistance, they have poor heat resistance. Although membranes made of polyamide are excellent in heat resistance, they cannot be sterilized with chlorine. The ultrafiltration membranes with which these problems are relatively well overcome are those made of polysulfon (Japanese Laid Open Patent Application (Kokai) No. 22508/75). However, polysulfon membranes have problems in solvent resistance and degradation of permeation efficiency due to the adsorption of solutes or the like. On the other hand, although the problem of solvent resistance was overcome by making the ultrafiltration membranes of a polyimide (Japanese Laid Open Patent Application (Kokai) No. 90098/82), such membranes have poor alkali resistance.
As a porous membrane with excellent heat resistance, chemical resistance and solvent resistance, polyphenylenesulfide (PPS) membranes have been proposed. PPS membranes include two groups, that is, homogeneous porous molded products prepared by molding PPS into a film or hollow fiber by melt-extrusion and by stretching and heat setting the film or hollow fiber (Japanese Laid Open Patent Application (Kokai) No. 6733/83 and Japanese Patent Publication (Kokoku) No. 44404/85), and heterogeneous porous molded products prepared by dissolving PPS under severe conditions, molding the resultant solution into a hollow fiber and coagulating the hollow fiber (Japanese Laid Open Patent Application (Kokai) No. 248202/85).
With the former method, it is difficult to control the pore size of the membrane and the produced membranes cannot have heterogeneous structures, although they can exhibit high permeability as ultrafiltration membranes. With the latter method, because of the high crystallinity of PPS, PPS can be dissolved in only very few solvents even under very severe conditions. Since porous membranes can only be prepared under these limited conditions, it is difficult to control the separation efficiency of the membranes, so that it is difficult to obtain the demanded properties.
On the other hand, it has been proposed to promote the durability of PPS porous membranes by oxidizing PPS so as to convert the sulfide bond into a sulfon bond (Japanese Laid Open Patent Application (Kokai) No. 225636/88). However, with this method, since the oxidization step is carried out after a porous membrane is prepared by the above-described method, it is difficult to control the separation efficiency. Further, in cases where partial oxidization of the sulfide bond is carried out, the polymer becomes a random copolymer, so that the reproducibility of the production process and the durability of the resulting membranes are not satisfactory.
Another major problem in ultrafiltration membranes is the change in the separation efficiency due to the adsorption of the solute on the surface of the membrane.
To reduce adsorption, methods of making the membrane hydrophilic are now being investigated. For example, a method in which an ether bond is introduced into polyvinylidene fluoride (J. Memb. Sci., 36, 257 (1988); Desalination, 70, 207 (1988)), a method in which N-methylolamide and methylenebisamide are introduced into polyacrylonitrile (PCT/US88/00261), and a method in which a polysulfon membrane is treated with an aqueous nonionic polymer solution such as methyl or ethyl cellulose, polyethyleneglycol, and polyvinyl alcohol (ICOM'87) have been proposed.
However, no membranes prepared by these method satisfy the durability requirements mentioned heretofore. Thus, a hydrophilic membrane having excellent durability is strongly demanded.